Cholesterol homeostasis in humans is essential to health; failure to regulate the balance of dietary cholesterol intake, cholesterol synthesis and cholesterol excretion as bile salts leads to hypercholesterolemia and atherosclerosis. Here we focus on the roles that the hepatic lipase (HL) and apoE-rich HDL play in the uptake of cholesterol by liver. Specifically, we postulate that this HDL particle is preferentially degraded by HL, a process that facilitates hepatic uptake of cholesterol from circulation for excretion. Our previous work has shown that the presence of apoE in model lipid systems and in HDL particles increases phospholipid (PL) hydrolysis 3-fold. Others have shown that hydrolysis of HDL-PL facilitates cholesteryl ester uptake by a nonendocytotic mechanism. To test our postulate we will use two general approaches: 1) We will study the hydrolysis of natural HDL particles from monkeys on atherogenic diets that vary in the type of dietary fat. This study, in collaboration with Dr. L. Rudel, will determine the effect of saturated, monounsaturated, and polyunsaturated fat on HDL particle composition and the ability of HL to attack natural HDL's of varying acyl and apoprotein content. We will then use HDL-like particles that model natural HDL substrates to better define the regulation of HL at the molecular level. These particles provide carefully controlled composition to determine the effect of lipid composition, particle size, and apoprotein content on lipid hydrolysis by HL. Concomitantly, we will use model monomolecular films of substrate by which we can carefully regulate the surface properties of the lipid film and study the mechanism of apoE activation. 2) We will determine the role HL plays in the uptake of HDL lipid by rat hepatocytes to better understand how apoE, HL, and the apoE receptor on liver cells interact in HDL metabolism. In this study we will test our hypothesis that apoE can interact with its receptor and with HL to effect HDL metabolism. We will also use monkey hepatocytes so that a single species is investigated and so that our studies with monkeys can be related to the atherogenic index found by Dr. Rudel. Together these studies will establish the mechanism of apoE regulation of HDL metabolism by HL and its role in lipid clearance by liver. In turn this will help elucidate the roles HL and apoE play in cholesterol homeostasis.